1. Field of the Invention
The present invention relates generally to a super twisted nematic liquid crystal display (STN LCD), and more particularly to a method and an apparatus for converting gradation data in the STN LCD, which provides color moving pictures of excellent quality.
2. Description of the Related Art
With the wide spread of multimedia services, various kinds of display apparatus for providing images and/or moving pictures have been developed. The most typical display apparatus is a cathode-ray tube (CRT), which has an important weakness that the CRT is a bulky tube. Accordingly, a liquid crystal display (LCD) and a plasma display panel (PDP) have been increasingly popularized in recent. Particularly, the LCD apparatus wins popularity in view of its thin and light properties
The LCD consists primarily of two glass plates, on which transparent electrodes are formed, with some liquid crystal material filled between them. When a predetermined current is applied to the transparent electrodes, the current arranges and rotates the liquid crystal material in a specific direction so as to control the quantity of lights transmitting the liquid crystal material, thus displaying images. The LCD is made with either a passive matrix or an active matrix display grid. The active matrix LCD is also known as a thin film transistor (TFT) display. The TFT LCD having a transistor located at each pixel intersection can provide high quality images because it drives each pixel using the transistor directly. However, the TFT-LCD has shortcomings that it requires high electric power and its unit price is very high.
In the meantime, with the rapid development of Internet technology in recent years, wireless Internet services, through which a user can use the Internet by using a mobile communication terminal such as PCS (personal communication system) or cellular phone, are developed and popularized. In addition, according as the International Mobile Telecommunications-2000 (IMT-2000) has becomes commercially available, various kinds of display apparatus capable of providing color moving pictures have been adapted to the mobile communication terminal. In general, the mobile communication terminal requires a specific display device that has a small size, a low unit price and low power consumption as well. Accordingly, inexpensive simple matrix TN or STN LCD is considered to be suitable for the mobile communication terminal. Especially, the STN-LCD is the most noticeable because it has wider viewing angle and higher contrast than those of the TN-LCD and is able to display images with large capacity.
However, the STN-LCD has a long response time to input data due to the influence of cumulative response. Accordingly, in case that a fast moving picture is displayed through the STN-LCD panel, image sticking phenomenon, that an image of a previous screen does not immediately disappear but is overlapped with a subsequent image, deteriorates vividness of the moving picture. To solve this problem, there has been proposed a method of appropriately converting gradation data provided to the STN-LCD.
FIG. 1 illustrates characteristic curves showing transmissivity varied as time passes when predetermined gray-scale voltages are applied to the STN-LCD. In FIG. 1, (a), (b) and (c) represent transmissivity variation characteristics obtained when voltages corresponding to the brightest gradation state and the darkest gradation state (a), voltages corresponding to the brightest gradation state and a medium-brightness gradation state (b), and voltages corresponding to the darkest gradation state and the medium brightness gradation stale (c) are applied to the STN-LCD, respectively. Referring to FIG. 1, in ease of the waveform (a), the transmissivity varies very fast according to the variation in the applied voltages, resulting in a delay time of up to 20–30 ms, whereas, in case of the waveforms (b) and (c), the transmissivity varies considerably slow according to the variation in the applied voltages, resulting in delay time of more than 100 ms, respectively. Of course, all of STN-LCDs do not have the same response delay time with the aforementioned one but have different ones depending on the manufacturers and the product specifications. In general, all of STN-LCDs have a relatively fast response characteristic when a gray scale level of video data being displayed is sharply changed, whereas, they show a relatively slow response characteristic when the gray scale level of video data is slightly varied, thus resulting in residual images. Accordingly, it is known that the response speed of the STN-LCD can be improved if the STN-LCD is adequately over-driven to increase time-sequential variation in the gradation voltage applied to the STN-LCD.
U.S. Pat. No. 5,347,294, U.S. Pat. No. 5,465,102 and U.S. Pat. No. 5,344,533 and Japanese Patent Publication No. 1995-129133 disclose methods for improving the response speed of the STN-LCD by converting gradation data provided to an LCD panel to adequately over-drive the LCD panel.
FIG. 2 is a block diagram for explaining the basic concept of the aforementioned patents. The configuration shown in FIG. 2 includes first and second frame memories 1 and 2 for storing gradation data, and a ROM table 3 for gradation conversion. Here, the first frame memory 1 stores gradation data of a previous one frame acquired from a video signal, whereas, the second frame memory 2 stores gradation data of a current one frame obtained from the video signal. The ROM table 3 uses the gradation data output from the first and second frame memories 1 and 2 as addresses to output predetermined gradation data corresponding to the addresses. The ROM table 3 stores appropriate gradation data corresponding to variations between the previous gradation data and the current gradation data. For instance, the ROM table 3 is set to output gradation data “100” when the previous gradation data is “000” and the current gradation data is “010”. In the above-described configuration, the gradation data of the previous frame obtained from the video signal is compared with the gradation data of the current frame to set the variation between the two gradation values large forcibly, so as to improve the response speed of the STN-LCD.
However, the aforementioned prior art has following problems:
1. The prior art technique generates new gradation data to be currently displayed based on the previous gradation data stored in the first frame memory 1 and the current gradation data stored in the second frame memory 2. Here, the previous gradation data stored in the first frame memory 1 is not the one that has been actually displayed through the STN-LCD previously but the original one obtained from the video signal. Accordingly, when the gradation data acquired from the video signal increases in stages as time passes, such as “000”, “0101”, “1011 ”, “100” . . . for example, as represented by waveform (a) in FIG. 3, the new gradation data obtained through gradation conversion will be also increased in stages, as shown by waveform (b), similarly to the waveform (a). That is, waveform (b) represents an example where the ROM table 3 is set to make a new gradation value “2” larger than the current one when the difference between two gradation values stores in the first and second frame memories 1 and 2, respectively, is “2”, and makes a new gradation value “1” larger than the current one when the difference is “1”.
Comparing the waveforms (a) with (b), their gradation value variations with the lapse of time are similar to each other. That is, gradation value variation between the original gradation data and the newly obtained gradation data with the lapse of time is set similar to each other. This occurs mostly when the time-sequential variation in the gradation data value increases or decreases in stages. In the prior art, however, since the gradation conversion is executed based on the gradation value obtained from the video signal, not based on the gradation value that has been previously displayed through the LCD, it is insufficient to improve the response speed effectively.
2. In the configuration of the prior art shown in FIG. 2, since the gradation conversion is carried out through the ROM table 3, it is necessary to store specific gradation data in advance in the ROM table 3 corresponding to the gradation data in the first and second frame memories 1 and 2. The specific gradation data stored in the ROM table 3 is an experimental value and variable depending on manufacturers or kinds of LCD panels, which causes difficulties in designing and constructing the ROM table. Especially, to meet the consumers demand for high-quality pictures in recent years, the number of colors to be displayed through the LCD panel reaches 65,000 to 260,000. The increase in the number of colors causes many difficulties in configuring the ROM table 3, which results in considerable restrictions on the application range and design flexibility of the apparatus employing the ROM table 3.